The present invention relates generally to a scanning device and a scanning method for causing a beam of light to sweep across a surface and. more particularly, to such a device and method in which the beam is raster scanned onto a flat stationary field.
Scanning devices for sweeping a light beam across a surface are used for a variety of reading and imaging applications. For example, color scanners may employ photoelectric sensors and color filters to analyze a color photograph on a point-by-point basis to produce color component electrical signals for a series of color separations. The signals may then be used to control the exposure of a photographic film or other imaging media which forms the color separations. The imaging media is exposed point-by-point to light modulated in accordance with the color component electrical signals such that the color separations reflect the color component densities at the scanned points on the original photograph. Similar light beam scanning operations are utilized to reproduce original images or paste-up images of printing or photographic plates for single color such as black and white reproduction. Additionally, scanning performed by sweeping a beam of light across a surface is utilized in facsimile and character recognition systems.
Light beam scanning devices have evolved from the simplest scanners wherein the image or imaging media to be scanned was curved and secured to an internal or external drum system. The drum was then mechanically moved to scan the image or imaging media by means of a stationary light beam typically generated by means of a laser. Optically, such systems are very simple. However, the image or imaging media must be secured to a rapidly moving drum which creates significant handling problems since a relatively bulky drum must not only be moved rapidly and with high precision but also with very smooth motion. In addition, such drum scanninq systems require movement over different distances, i.e., movement scaling, as the image or imaging media sizes change. Movement scaling greatly increases the precision and dynamic range required of the combined fast and slow scan motion control system of the scanner.
Light beam scanning systems were improved by moving the light beam in the fast scan direction such that the image or imaging media support drum only had to move in the slow scan direction. While this was a big improvement over preceding scanner systems, curved image or imaging media holders were still required and the slow scan and fast scan motion systems, although now separated, retained the problem of having to be scaled for size changes.
A more recent improvement is the use of a flat field optical system which allows the light beam to fast scan a line in a flat image plane while still providing slow scan movement by moving the image plane. While these systems simplify media holding, since the media is now held flat, they still require two widely separated precision motion systems since the media is still moved to produce the slow scan function. Such systems also retain all the problems of separated motion systems which must be scaled for size changes.
Most flat field scanners use f theta lens systems which convert angular motion of the scanner light beam into a linear motion along a flat line at the image plane. Ideally, one would perform both the fast and slow scans on a flat stationary image field to completely simplify the image or imaging media holder for scanning operations. However, when both the slow and fast scan motions are performed together to raster scan a flat stationary image plane or field, significant distortions are introduced into the scanned image.
Accordingly, there is a need for a stationary flat field light beam scanner which provides high image resolution with low image distortion.